Resonant circuits have long been used to amplify small signals. A regenerative circuit employs positive feedback between the input and output of an amplifier to significantly increase the signal strength. Positive feedback occurs when the phase of the feedback signal is in phase with the input signal and thus reinforces the signal strength. The addition of a resonator to the feedback loop serves to filter the signal so that only the frequency of the resonator is amplified. This type of circuit is well-known and was patented by Edwin Armstrong in 1914 for use as a high-sensitivity radio receiver. Increases of 1,000 to 100,000 times for the gain and the passive quality factor (Qstat) of the circuit are achievable due to the positive feedback. The Q factor is a measure of the frequency selectivity of a resonant circuit and is defined as f0/Δf, where f0 is the center frequency and Δf is the bandwidth between the −3 dB frequencies. Regenerative circuits with dielectric resonators in the feedback loop have been used to construct low phase noise microwave oscillators.
What is needed is a regenerative feedback resonant circuit that measures a transient response due to perturbations in electromagnetic properties of various materials.